Strategic Cell-Cycle Regulatory Features That Provide Mammalian Cells with Tunable G1 Length and Reversible G1 Arrest

Transitions between consecutive phases of the eukaryotic cell cycle are driven by the catalytic activity of selected sets of cyclin-dependent kinases (Cdks). Yet, their occurrence and precise timing is tightly scheduled by a variety of means including Cdk association with inhibitory/adaptor proteins (CKIs). Here we focus on the regulation of G1-phase duration by the end of which cells of multicelled organisms must decide whether to enter S phase or halt, and eventually then, differentiate, senesce or die to obey the homeostatic rules of their host. In mammalian cells, entry in and progression through G1 phase involve sequential phosphorylation and inactivation of the retinoblastoma Rb proteins, first, by cyclin D-Cdk4,6 with the help of CKIs of the Cip/Kip family and, next, by the cyclin E-Cdk2 complexes that are negatively regulated by Cip/Kip proteins. Using a dynamical modeling approach, we show that the very way how the Rb and Cip/Kip regulatory modules interact differentially with cyclin D-Cdk4,6 and cyclin E-Cdk2 provides to mammalian cells a powerful means to achieve an exquisitely-sensitive control of G1-phase duration and fully reversible G1 arrests. Consistently, corruption of either one of these two modules precludes G1 phase elongation and is able to convert G1 arrests from reversible to irreversible. This study unveils fundamental design principles of mammalian G1-phase regulation that are likely to confer to mammalian cells the ability to faithfully control the occurrence and timing of their division process in various conditions.

真核细胞周期连续阶段之间的转换，由特定组合的细胞周期蛋白依赖性激酶（cyclin-dependent kinases, Cdks）的催化活性所驱动。然而，其发生与精确时序受到多种机制的严格调控，其中包括细胞周期蛋白依赖性激酶与抑制性/适配蛋白（CKIs）的结合。本研究聚焦于G1期时长的调控：多细胞生物的细胞需在G1期末决定是否进入S期，或是停止增殖，最终通过分化、衰老或凋亡以契合宿主的稳态规则。在哺乳动物细胞中，G1期的进入与推进涉及成视网膜细胞瘤Rb蛋白的依次磷酸化与失活——首先由细胞周期蛋白D-Cdk4/6在Cip/Kip家族CKIs的辅助下完成，随后由受Cip/Kip蛋白负调控的细胞周期蛋白E-Cdk2复合物介导。本研究采用动力学建模方法，证实Rb与Cip/Kip调控模块分别与细胞周期蛋白D-Cdk4/6、细胞周期蛋白E-Cdk2产生差异化相互作用的方式，赋予哺乳动物细胞一种高效调控手段，可实现对G1期时长的高精度控制以及完全可逆的G1期阻滞。与此一致的是，任一调控模块的功能失调都会阻碍G1期延长，并可将G1期阻滞从可逆状态转变为不可逆状态。本研究揭示了哺乳动物G1期调控的核心设计原则，这类原则或可使哺乳动物细胞在多种条件下均能精准调控细胞分裂过程的发生与时序。